The present invention relates generally to fullerenes, and more particularly, to the recovery of fullerenes from the carbon matrix co-produced with the fullerenes. Still more particularly, the invention relates to specific conditions for the reductive extraction of fullerenes, regardless of molecular electronic structure, from a non-fullerene carbon matrix into a solvent.
Fullerenes are the molecular form of the element carbon. They are distinguished by their multi-faceted, closed structure, where the carbon-carbon bonds form a framework of hexagons and pentagons that resembles the familiar hexagon/pentagon surface of a soccer ball. The number and positioning of the hexagons and pentagons can vary, within both constraints that exactly 12 pentagons and that an even number of carbon atoms be present. It happens that the essentially spherical molecule formed by sixty carbon atoms (C60) comprises a particularly stable combination of hexagons and pentagons and is the most widely studied fullerene to date. In general, more than one arrangement of the hexagons and pentagons is possible, leading to a great variety of possible isomers for any particular number of carbon atoms in a fullerene. To help specify a particular fullerene isomer, the symmetry group name to which that isomer belongs is affixed to the molecular formula, but even this is imperfect as it is common for many isomers belonging to the same point group to be present for any particular number of carbon atoms.
There are two widely practiced methods for making fullerenes. The first involves evaporating carbon atoms from graphite and cooling them in such a way that some of them assemble into fullerene molecules. The second involves burning a hydrocarbon in a fuel-rich flame, and adjusting the conditions such that some of the unburnt carbon atoms condense into fullerenes. For both methods, even under optimal conditions, not all of the evaporated carbon atoms end up in fullerene molecules. The remainder condenses into carbon structures that are not molecular in nature, which we will refer to as soot. While C60—Ih forms a significant fraction of the total fullerene production, the fullerene molecules that are produced can have more than three hundred carbon atoms.
There are three established methods for the extraction of fullerenes from the surrounding carbon matrix: sublimation, solvent extraction, and, for small-bandgap fullerenes, altering the charge of the fullerene to render it soluble. Other methods for separating fullerenes from the carbon matrix involving the formation of covalent bonds between an extracting reagent and the fullerenes have been proposed, but incur the additional and usually difficult step of subsequently undoing the bond formation in order to achieve pure fullerenes. These methods are generally not practiced, except when the fullerenes are created solely for the purpose of preparing a specific fullerene derivative.
Fullerenes can be sublimed from the carbon matrix at temperatures exceeding ˜400° C. However, sublimation is expensive to perform, and many fullerenes react with the matrix before subliming. Also, fullerenes trapped within matrix carbon particles cannot sublime out. For these reasons, sublimation is rarely practiced.
Solvent extraction is the most commonly practiced method for recovering fullerenes from the carbon matrix. The fullerene-containing matrix is contracted with a solvent in which certain fullerenes are soluble, such as toluene. C60—Ih can be recovered in this manner, as are several other fullerenes with isolated pentagons, including but not limited to, C70-D5h, C76-D2, C78-C2v′, C78-C2v″, C78-D3, C80-D2, C84-D2d and C84-D2. However, the solvents in which the fullerenes have the best solubility are solvents that intercalate readily into the fullerene lattice, and also usually have very low vapor pressures, making it very difficult to remove the best solvents from the extracted fullerenes. For example, even though C60 is over ten times as soluble in 1-methylnaphthalene as in toluene (Ruoff et al., 1993), 1-methylnaphthalene is virtually never used to extract fullerenes from soot, while toluene is employed routinely. Physical means may be used to break apart the soot particles trapping fullerenes inside, but there is additional cost associated with such particle disruption. Once dissolved, the fullerenes can be recovered by removing the solvent, or by diluting the solvent with another solvent in which the fullerenes have little solubility, causing the fullerenes to precipitate. However, even relatively low-boiling solvents invariably co-crystallize (intercalate) with the precipitated fullerenes to some extent. The intercalation is most pronounced with the aromatic solvents that are also the best solvents for fullerenes. It is believed to be due to the same van der Waals attractions between the solvent's pi electrons and the fullerene that allow the liquid to be an effective solvent for the fullerene.
Also, there are many fullerenes that are predicted to be thermodynamically stable, and observable by mass spectrometric analysis of the soot, but which are not found among the fullerenes that are extracted using the method described above. These also include fullerenes that have been synthesized to include a single metal atom, such as a lanthanide, actinide or alkaline earth metal atom, known as endohedral metallofullerenes. These fullerenes are known as “small-bandgap fullerenes” because of the small or zero energy difference between their filled and unfilled molecular orbitals (Diener and Alford, 1998). While certain endohedral metallofullerenes, most commonly those with a C82 fullerene cage for the metal atom, do exhibit limited solubility in the same solvents that are useful for extraction of C60 etc., these constitute a minority of all endohedral fullerenes formed (though the vast majority of endohedral metallofullerene research is performed on those of the C82 fullerene). Aside from this exception, small bandgap fullerenes are not extracted by contacting with solvents. Hence, it is desirable to provide a method for recovering these previously unrecoverable fullerenes, including metallofullerenes. It is further desired to provide a fullerene isolation method that is simple and easy to execute, and that does not disrupt or affect the subject fullerenes.
Certain organic liquids containing an amine group which are commonly used as solvents exhibit a complicated behavior towards fullerenes, including the small bandgap fullerenes. Contacting fullerene containing soot with aniline or pyridine, e.g., results in fullerenes, including small bandgap fullerenes, in the liquid phase. The best explanation of the interaction between aniline (or pyridine) and the fullerenes is that of a charge-transfer complex (Sibley et al., 1995, e.g.), rather than a simple solute-solvent relationship. Since the aniline (or pyridine) cannot be separated from the fullerenes after they have been contacted, this interaction is better understood as a reactive extraction, and includes the formation of (weak) bonds between the amine and the fullerene. Other methods for reactive extraction of giant fullerenes from soot through Diels-Alder reactions are also known (Beer et al., 1997). Chemical derivatization of small bandgap fullerenes with serinol has also been disclosed (Diener and Alford, 2001). The extraction of small bandgap fullerenes and endohedral metallofullerenes with diethyl bromomalonate and 2-bromo-2-phenylacetophenone has also been demonstrated (Bolskar and Alford, 2003a).
It is also known that additional fullerenes could be extracted from the soot matrix by reduction to an anionic charge state (Diener and Alford, 1998; 2001; 2003), or chemical derivatization (ibid; Diener and Alford, 2001). In relation to this prior art, the invention was based on object of further increasing the amount of fullerenes that can be extracted and subsequently purified from the carbon matrix. From the wide variety of conditions possible within the methods given by Diener and Alford (2001), this invention discloses methods that are more effective at recovering the generally insoluble fullerenes and metallofullerenes from the soot. The methods of this invention also extract a much greater amount of soluble fullerenes, including C60, from the soot than can be extracted by contacting with solvents. Furthermore, this invention provides for the use of solvents which are easily separated from the fullerenes following extraction.